Direct and indirect effects of grazing on the phytoplankton seasonal succession in an oligotrophic lake

We examined the simultaneous dynamics of phytoplankton and zooplankton in an oligotrophic lake, as well as the algal response to experimental manipulations of herbivore population structure and density. The seasonal succession of phytoplankton is characterized by a shift in dominance from eukaryotic species to prokaryotic cyanobacteria, as in a eutrophic lake. This unusual pattern for an oligotrophic lake is related to the release of zooplankton phosphorus, rather than to the amount of total phosphorus. The highest estimated values of this released phosphorus occur during the co-dominance of small-sized species and of one calanoid population, both under natural and experimental conditions. Our experimental results clearly reveal that a blue-green species replaces non-blue-green ones when environmental conditions render low or intermediate values for released phosphorus, irrespective of the grazing pressure. The absence of predators or high levels of released phosphorus stimulate non-blue-green growth. These results agree with the successional pattern observed for the algal assemblage under natural conditions. Nutrient recycling plays an essential role in the maintenance of the metabolism of the pelagic system, and therefore the prevalence of a topdown regulation mechanism as proposed for such low-nutrient environments should be reconsidered.